The present invention relates to rotary cutters for successively cutting a continuous moving material, such as, paper, sheeting or tube, and more particularly the invention relates to a system for controlling the speed of a rotary cutter driving DC motor in accordance with a rotary cutter speed control pattern which is dependent on the relation between the desired material-cutting length and the length of the rotary cutter circumference.
The rotary cutters of the above type now in use most widely are divided into those which are controlled mechanically and others which are controlled electronically. In the case of the mechanically controlled rotary cutter, the rotary cutter is connected to the power source of a material feeding mechanism through a speed change gear and a crank chain so that the rotary cutter and the material feed mechanism are driven from the same power source. In other words, a change in the material-cutting length is effected by changing the change gear ratio.
Also during the periods of cutting operation, the cutter speed and the material travel speed must be made equal to each other, and consequently the material travel speed is made equal to the cutter speed by the nonuniform motion of the crank chain.
A disadvantage of this type of mechanically controlled rotary cutter is that since the speed changing operation can be effected only gradually, when effecting a change in the material-cutting length, the material is lost during the time that the length is being changed, that during the periods of acceleration and deceleration of the material travel speed, the material-cutting length is changed by the difference in mechanical deflection between the feed mechanism and the rotary cutter, and that a cutting error is caused by the slip between the feed mechanism and the material.
On the other hand, while a large part of the deficiencies of the mechanical type have been overcome by the electronically controlled rotary cutter, the electronically controlled rotary cutter has the following disadvantages.
The conventional electronically controlled rotary cutters are based on the method in which material travel detection pulses are subtractively applied to a register and also a number of pulses corresponding to the material-cutting length are also applied additively to the register each time the cutter moves past the cutting end point. At the same time, a number of pulses corresponding to one rotation of the rotary cutter are subtractively applied to the register, and the content of the register is converted into a DC voltage of the opposite sign which in turn is applied in proportion to the material travel speed to the DC voltage constituting the forward running speed command for the cutter. Consequently, only when the resulting sum has the polarity which rotates the cutter in the forward direction, the sum is applied as a speed reference to a speed controller of the cutter driving DC motor.
With this method, since the pulses corresponding to the material-cutting length and those corresponding to one rotation of the cutter are applied upon completion of cutting, excepting where the number of pulses corresponding to one rotation of the cutter is close to that corresponding to the material-cutting length, the rotary cutter is subjected, irrespective of the material travel speed, to rapid acceleration or deceleration which is dependent on the current limitation of the servomotor and consequently the maximum torque is always exerted on the mechanical parts.
This tends to considerably deteriorate the durability of the mechanical parts.
Also, since a large power is handled in the electronically controlled rotary cutter, the cutter is usually controlled by a DC motor and a thyristor Ward-Leonard control system. However, it is the usual practice to increase the response speed at the sacrifice of the loop gain due to the necessity to control the high speeds with the limited frequencies, and consequently the above-mentioned control system is not able to provide the necessary operating speeds as well as the acceleration and deceleration speeds, thus making it impossible to compensate for the decrease in the gain by a digital circuit. This results in deteriorated accuracy of the servomotor. On the other hand, since the content of the register is converted into a DC voltage of the opposite sign and combined with the material travel speed voltage so that the resulting sum is applied as an input to the rotary cutter driving servomotor only when its value is of the polarity which rotates the cutter in the forward direction, if the sum is within the range which does not cause the cutter to rotate in the forward direction, a zero input is applied as the servo input irrespective of the content of the register.
Thus, if, in these conditions, a zero point drift occurs in the servomotor, despite the fact that the cutter must be at rest, the cutter is slowly rotated in one or the other direction.
In the case of long lengths, this has the effect of reducing the effective follow-up time and thereby deteriorating the accuracy.
Further with this system, taking one example where there is no need to take into consideration the frictional torque of the rotary cutter and where the material-cutting length is greater than the length of the cutter circumference, the DC voltage derived by converting the register content is so set that when the register content corresponds to the length of the cutter circumference, the DC voltage obtained by converting it becomes equal to the maximum travel speed voltage of the material which is permitted by the maximum acceleration/deceleration of the rotary cutter.
This means that the feedback quantity of error cannot be set to obtain the optimum results, thus resulting in deteriorated accuracy. With this system, the follow-up speed of the servomotor also decreases exponentially with decrease in the register content, namely, when the register content decreases to one half, the follow-up speed of the servomotor also decreases to one half, and the follow-up speed decreases to one fourth when the register content decreases to one fourth, and so on. Thus, as compared with the systems in which the servomotor is caused to follow up linearly until the error is decreased sufficiently, far great follow-up time is required. Even if the follow-up is not sufficient, the error in the previous cutting is retained in the register so that the material can be cut to the desired length during the next cutting operation. However, the rentention of such steady-state error tends to cause the error to be changed by a change in the speed as well as a change in the material-cutting length, thus causing the actual error.